Substrate homogenization affects survival and fitness in the lowland stream caddisflies Micropterna sequax and Potamophylax rotundipennis: A mesocosm experiment

Loss of substrate heterogeneity or patchiness is common in lowland streams with disturbed hydrological regimes. At the reach scale, peak discharges tend to homogenize the stream bed and decrease the availability of specific microhabitat types. This spatial shift in habitats toward a more homogeneous landscape could have large negative effects on species that perform essential ecosystem processes. We conducted an aquatic mesocosm experiment to test the effect of habitat homogenization on survival and fitness of 2 species of Trichoptera (Micropterna sequax and Potamophylax rotundipennis). We used caddisflies as model organisms because of their abundance in lowland streams, their representativeness of the total shredder community, and their important role in leaf-litter decomposition. We reared larvae in artificial recirculating channels containing leaf and sand patches in 3 spatial configurations, differing in homogeneity of substrates, varying from few large patches to many small patches. We used emergence rate as a measure of survival and biomass and wing span of the adults as fitness correlates. For M. sequax, survival was lower in the homogeneous than in the heterogeneous configurations, but patch configuration did not affect fitness. For P. rotundipennis, spatial configuration of the patches did not affect survival, but the longest forewings for both males and females were found in the homogeneous configuration. Our results suggest that both species experience intraspecific resource competition arising from the spatial distribution of patches, expressed as an investment in wing development (e.g., dispersal capacity) in P. rotundipennis and resulting in lower survival rates in M. sequax. Our results indicate the importance of knowledge of trait-based responses and highlight the effects of the configuration of stream bottom substrate for its inhabitants on microscale.